The aim of this work is to evaluate the capability of tomato plants to use different Fe sources, such as Fe citrate, Fe phytosiderophores, and Fe complexed by a water-extractable humic substances (Fe-WEHS) also in re- lation to physiological and molecular adaptations induced by these complexes at the root level. Tomato plants acquired higher amounts of Fe from Fe-WEHS than from the other two sources and this phenomenon occurred only when the treatment lasted 24 h. The higher acquisition of Fe from Fe- WEHS than other sources depended on a reductive mecha- nism and on rhizosphere acidification and appeared to be due neither to a higher apoplastic loading nor to a higher resistance of WEHS to microbial degradation. Supply of the different Fe complexes to deficient plants induced a tran- sient upregulation of Fe(III)-chelate reductase (LeFRO1) and Fe transporter genes, LeIRT1 and LeIRT2. In Fe- WEHS-fed plants, where a quicker and higher upregulation of these genes was evident, a coordination in the expression of LeFRO1, LeIRT1, and LeIRT2 genes occurred already after 1 h treatment when the amount of Fe acquired by the plants from the three sources was similar. Iron from Fe- WEHS could be efficiently acquired in a mixture of natural Fe complexes possibly occurring in the rhizosphere. This phenomenon is due to an altered expression of Fe uptake- related genes and to the root capacity to create favorable conditions for the micronutrient uptake into the rhizosphere.
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